Preheat the oven to 350°.
Spray a large ovenproof skillet with non-stick spray and over medium-high heat, cook onion until translucent. Add the spinach and garlic and cook for an additional minute or until the spinach is all wilted.
Dump the mixture into a large bowl and add the ground beef, egg, bread crumbs, 1/2 of the lemon juice, oregano, cumin, salt and 1/2 c. feta. Combine the mixture with your hands until everything is blended, being sure to not overwork the meat. Roll the mixture into 1" meatballs.
Give the pan another coat of non-stick spray and add the meatballs, cooking for one minute on each side, rotating until all sides are cooked.
Top the meatballs with the chopped veggies and bake at 350° for 30-40 minutes or until the meatballs are no longer pink in the center. Cooking times will vary on the size of meatballs.
Remove the skillet from the oven and top with the remaining 1/2 c. feta and remaining lemon juice. Give a rough stir and serve immediately.
*Serving suggestion - Serve with a spinach salad or a side of quinoa tossed with olive oil, lemon juice, salt, pepper, tomatoes and feta.